What if an account is using my registered trademark as its username?

Trademark violations may involve using a company or business name, logo or other trademark-protected materials in a manner that may mislead or confuse others with regard to its brand or business affiliation.

What is not a violation?

Using another's trademark in a way that has nothing to do with the product or service for which the trademark was granted is not a violation of Instagram's trademark policy. Note that Instagram usernames are provided on a first-come, first-served basis and may not be reserved.